I have (on other cars) removed a sticky starter solenoid with starter left in the car and taken the solenoid apart, cleaned and lubed and it worked like new. Water gets in and corrodes the parts inside and also the grease dries up. After taking it apart, a good cleaning with emery cloth and some grease will restore it's function. I don't know if the solenoid on the 123 starter is removable or can be taken apart. That's the first thing I would try before pulling a starter if it were.

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85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked